Java8新特性----Lambda表达式详细探讨
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Java8新特性
Lambda表达式
Lambda是一个匿名函数，可以理解为一段可以传递的代码（将代码像数据一样传递）；可以写出更简洁、更灵活的代码；作为一种更紧凑的代码风格，是Java语言表达能力得到提升
入门演示
案例1
public class TestMain
{
//使用匿名内部类完成比较
@Test
public void test()
{
//比较器 匿名内部类，创建该接口的一个实现类
Comparator<People> com=new Comparator<People>() {
@Override
public int compare(People o1, People o2) {
return o1.getAge()-o2.getAge();
}
};
//TreeSet的特点是可排序、不重复
TreeSet<People> ts=new TreeSet<>(com);
ts.add(new People("大忽悠",20));
ts.add(new People("小忽悠",18));
ts.add(new People("大大大",22));
ts.add(new People("小朋友",17));
ts.forEach(System.out::println);
}
//lambda表达式替代匿名内部类
@Test
public void test1()
{
//比较器
Comparator<People> com=(p1,p2)-> p1.getAge()-p2.getAge();
//TreeSet的特点是可排序、不重复
TreeSet<People> ts=new TreeSet<>(com);
ts.add(new People("大忽悠",20));
ts.add(new People("小忽悠",18));
ts.add(new People("大大大",22));
ts.add(new People("小朋友",17));
ts.forEach(System.out::println);
}
}
如何解决 cannot be cast to java.lang.Comparable问题？
如何解决 cannot be cast to java.lang.Comparable问题？
案例2
要求对下面的代码进行优化:
public class TestMain
{
List<People> peopleList= Arrays.asList(
new People("1号",18,3000),
new People("2号",21,4000),
new People("3号",19,5000),
new People("4号",20,3500)
);
//获取年龄大于18的
public List<People> getAgeOver18()
{
List<People> list=new ArrayList<>();
for (People p:peopleList)
{
if(p.getAge()>18)
list.add(p);
}
return list;
}
//获取工资大于3000的
public List<People> getMoneyOver3000()
{
List<People> list=new ArrayList<>();
for (People p:peopleList)
{
if(p.getMoney()>3000)
list.add(p);
}
return list;
}
@Test
public void test()
{
List<People> ageOver18 = getAgeOver18();
ageOver18.forEach(System.out::println);
System.out.println("======================================");
List<People> moneyOver3000=getMoneyOver3000();
moneyOver3000.forEach(System.out::println);
}
}
优化方式一 : 策略设计模式
声明一个接口MyPrediect
public interface MyPrediect<T>
{
public boolean test(T t);
}
接口的实现类一FilterPeoAge，负责过滤年龄:
public class FilterPeoAge implements MyPrediect<People>{
@Override
public boolean test(People people) {
return people.getAge()>18;
}
}
接口实现类二FilterPeoMoney，负责过滤金钱
public class FilterPeoMoney implements MyPrediect<People>{
@Override
public boolean test(People people) {
return people.getMoney()>3000;
}
}
测试演示:
public class TestMain
{
List<People> peopleList= Arrays.asList(
new People("1号",18,3000),
new People("2号",21,4000),
new People("3号",19,5000),
new People("4号",20,3500)
);
public List<People> FilterPeo(List<People> list,MyPrediect<People> mp)
{
List<People> peopleList=new ArrayList<>();
for (People p:list)
{
if(mp.test(p))
peopleList.add(p);
}
return peopleList;
}
@Test
public void test()
{
List<People> peopleList = FilterPeo(this.peopleList, new FilterPeoAge());
peopleList.forEach(System.out::println);
System.out.println("===========================");
List<People> peopleList1 = FilterPeo(peopleList, new FilterPeoMoney());
peopleList1.forEach(System.out::println);
}
}
当我们还需要安装某个策略进行过滤时，只需要实现接口，完成相应策略过滤逻辑编写即可
优化方式二: 策略设计模式+匿名内部实现接口，减少创建实体类的麻烦
public class TestMain
{
List<People> peopleList= Arrays.asList(
new People("1号",18,3000),
new People("2号",21,4000),
new People("3号",19,5000),
new People("4号",20,3500)
);
public List<People> FilterPeo(List<People> list,MyPrediect<People> mp)
{
List<People> peopleList=new ArrayList<>();
for (People p:list)
{
if(mp.test(p))
peopleList.add(p);
}
return peopleList;
}
@Test
public void test()
{
List<People> peopleList = FilterPeo(this.peopleList, new MyPrediect<People>() {
@Override
public boolean test(People people) {
return people.getAge()>18;
}
});
peopleList.forEach(System.out::println);
System.out.println("===========================");
List<People> peopleList1 = FilterPeo(peopleList, new MyPrediect<People>() {
@Override
public boolean test(People people) {
return people.getMoney()>3000;
}
});
peopleList1.forEach(System.out::println);
}
}
优化方式三: lambda表达式
public class TestMain
{
List<People> peopleList= Arrays.asList(
new People("1号",18,3000),
new People("2号",21,4000),
new People("3号",19,5000),
new People("4号",20,3500)
);
public List<People> FilterPeo(List<People> list,MyPrediect<People> mp)
{
List<People> peopleList=new ArrayList<>();
for (People p:list)
{
if(mp.test(p))
peopleList.add(p);
}
return peopleList;
}
@Test
public void test()
{
List<People> peopleList = FilterPeo(this.peopleList, (people)-> people.getMoney()>4000);
peopleList.forEach(System.out::println);
}
}
优化方式四: stream流
public class TestMain
{
List<People> peopleList= Arrays.asList(
new People("1号",18,3000),
new People("2号",21,4000),
new People("3号",19,5000),
new People("4号",20,3500)
);
public List<People> FilterPeo(List<People> list,MyPrediect<People> mp)
{
List<People> peopleList=new ArrayList<>();
for (People p:list)
{
if(mp.test(p))
peopleList.add(p);
}
return peopleList;
}
@Test
public void test()
{
peopleList.stream().filter(people -> people.getMoney()>3000).limit(2).forEach(System.out::println);
}
}
Lambda语法
- 操作符：->
- 左侧：参数列表
- 右侧：执行代码块 / Lambda 体
即lambda是对接口的抽象方法的实现，可能有人会问，如果接口中抽象方法存在多个，那lambda是对哪个抽象方法的实现呢?
其实lambda需要一个函数式接口的支持，即当前接口只有一个抽象方法
语法格式一 : 无参数，无返回值
@Test
public void test()
{
Runnable runnable = new Runnable() {
@Override
public void run() {
System.out.println("方法执行中.....");
}
};
runnable.run();
System.out.println("----------------------");
Runnable r1=()-> System.out.println("r1执行中....");
r1.run();
}
注意: 局部内部类与局部变量
局部内部类在JDK8之前只能使用成员变量和被final修饰的局部变量。JDK8之后，局部内部类如果使用局部变量那么局部变量默认被final修饰，但如果局部变量被重新赋值，那么局部内部类将不能在使用。具体看下例子：
public void show() {
int j = 0; //jdk1.8后默认被final修饰。1.8之前局部内部类只能访问成员变量，和被final修饰的局部变量
class A{
public void showA<以上是关于Java8新特性----Lambda表达式详细探讨的主要内容，如果未能解决你的问题，请参考以下文章
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